1. Field of the Invention
The present invention relates to a head driver, and more particularly, it relates to a head driver for driving a control head of a video tape recorder for recording a control signal such as a program search signal, a signal for controlling a tape travelling speed or the like in a control track of a video tape, for example.
2. Description of the Background Art
FIG. 1 is a circuit diagram showing a conventional control head driver of a video tape recorder. A signal received in an input terminal 1 is directly supplied to a base of an NPN transistor Q1 and a current mirror 3. The signal received in the input terminal 1 is also supplied to a base of an NPN transistor Q2 and a current mirror 4 through an inverter 2. The transistors Q1 and Q2 have collectors which are connected to source voltage V.sub.CC. A control head coil 5 for recording a control signal in a video tape is connected between emitters of the transistors Q1 and Q2.
The current mirror 3 is formed by an NPN transistor Q3 and a diode D1. The transistor Q3 has a collector which is connected to the emitter of the transistor Q2, an emitter which is grounded, and a base which is connected to the input terminal 1 as well as to an anode of the diode D1. The transistors Q2 and Q3 form a totem pole type driver. The diode D1 has a cathode grounded.
The current mirror 4 is formed by an NPN transistor Q4 and a diode D2. The transistor Q4 has a base which is connected to an output of an inverter 2 as well as to an anode of the diode D2, a collector which is connected to the emitter of the transistor Q1 and an emitter which is grounded. The transistors Q1 and Q4 form a totem pole type driver. The diode D2 has a cathode grounded.
The current mirrors 3 and 4 enter operable states (hereinafter referred to as selected states) upon supply of a high-level signal, and start operation when current is supplied from a current mirror 6. Upon supply of a low-level signal, on the other hand, the current mirrors 3 and 4 are disabled not to operate even if current is supplied from the current mirror 6.
The current mirror 6, which is adapted to supply current to the current mirrors 3 and 4, is formed by PNP transistors Q5 to Q8. The transistors Q5 to Q7 have bases which are connected with each other, and emitters which are connected to the source voltage V.sub.CC. The transistor Q5 has a collector which is connected to the base of the transistor Q3. The transistor Q6 has a collector which is connected to the base of the transistor Q4. The transistor Q8 has a base and an emitter which are connected to the collector and the base of the transistor Q7 respectively, and a collector grounded.
The NPN transistor Q9 has a collector which is connected to the collector of the transistor Q7 through a resistor R1, a base which is connected to a control signal input terminal 7, and an emitter grounded. In response to a control signal from the control signal input terminal 7, the transistor Q9 activates or disables the current mirror 6.
FIG. 2 shows voltage waveforms at the respective terminals of the circuit shown in FIG. 1. The operation of the circuit shown in FIG. 1 will now be described with reference to FIG. 2. It is assumed here that an input signal A received in the input terminal 1 and a control signal B received in the control signal input terminal 7 are both at a low level. In this case, the transistor Q2 is in an ON state and the transistor Q1 is in an OFF state, while the current mirror 4 is in a selected state and the current mirror 3 is disabled. Since the control signal B is at a low level, the transistor Q9 is turned off, the current mirror 6 is in a disabled state, and the current mirror 4 performs no operation. Thus, no current flows through the control head coil 5, whereby potentials at both ends C and D of the control head coil 5 are substantially at the source voltage V.sub.CC.
It is assumed that only the input signal A is changed from a low level into a high level at a time t1. Then, the transistor Q2 is turned off and the transistor Q1 is turned on, while the current mirror 3 enters a selected state and the current mirror 4 is disabled. The control signal B remains at a low level. Thus, no current flows through the control head coil 5 similarly to the above case, whereby the potentials at the ends C and D of the control head coil 5 are substantially at the source voltage V.sub.CC.
It is assumed that the control signal B is changed from a low level into a high level at a time t2. The input signal A remains at a high level a the time t2. Then, the transistor Q9 is turned on and current is supplied to the current mirror 6 through the resistor R1. The current mirror 3 starts operation to flow current in a direction from the end D to the end C of the control head coil 5. At this time, however, back electromotive force is caused by abrupt current change, so that no current instantaneously flows through the control head coil 5. In an instant of the operation of the current mirror 3, therefore, the potential at the point C is pulled down to GND. After the current flowing through the control head coil 5 becomes sufficient, the potential at the point C is stabilized at a level which is smaller than the source voltage V.sub.CC by voltage drops caused by the transistor Q1 and the control head coil 5. On the other hand, the potential at the point D is stabilized at a level which is smaller than the source voltage V.sub.CC by a voltage drop caused by the transistor Q1. In this mode, the control track of the video tape is magnetized by the current flowing in the direction from the end D to the end C, to record a control signal of prescribed polarity.
It is assumed that only the control signal B is changed from a high level into a low level at a time t3. Then, the transistor Q9 is turned off, the current mirror 6 is disabled and no current is supplied to the current mirror 3. Therefore, the current flowing through the control head coil 5 in the direction from the end D to the end C is cut off to cause abrupt current change. In the instant when the control signal B is changed from a high level into a low level, therefore, back electromotive force is caused to continuously make the current flow in the direction from the end D to the end C, whereby the potential at the point C exceeds the source voltage V.sub.CC. After a lapse of a constant period, no current flows through the control head coil 5 and the potentials at the points C and D substantially reach the source voltage V.sub.CC.
It is assumed that the input signal A is changed from a high level into a low level at a time t4. Then, the transistor Q2 is turned on and the transistor Q1 is turned off, while the current mirror 3 is disabled and the current mirror 4 enters a selected state. Since the control signal B is at a low level, no current is supplied to the current mirrors 3 and 4 at this time, and hence no current flows through the control head coil 5. The ends C and D of the control head coil 5 are at the same potentials.
It is assumed that only the control signal B is changed into a high level at a time t5. Then, the transistor Q9 is turned on and the current mirror 6 is activated, while the current mirror 4 starts operation to flow current in a direction form the end C to the end D of the control head coil 5. At this time, back electromotive force is caused by abrupt current change, so that almost no current instantaneously flows through the control head coil 5. In an instant of the operation of the current mirror 4, therefore, the potential at the point D is pulled down to GND. After the current flowing in the direction from the end C to the end D of the control head coil 5 becomes sufficient, the potential at the point D is stabilized at a level which is smaller than the source voltage V.sub.CC by voltage drops caused by the transistor Q2 and the control head coil 5. On the other hand, the potential at the point C is stabilized at a level which is smaller than the source voltage V.sub.CC by a voltage drop caused by the transistor Q2. In this mode, a control signal which is reverse in polarity to the above is recorded in the video tape.
It is assumed that only the control signal B is changed from a high level into a low level at a time t6. Then, the transistor Q9 is turned off, the current mirror 6 is disabled, and no current is supplied to the current mirror 4. Therefore, the current flowing through the control head coil 5 in the direction from the end C to the end D is cut off to cause abrupt current change. In the instant when the control signal B is changed from a high level into a low level, therefore, back electromotive force is caused to continuously make the current flow in the direction from the end C to the end D, whereby the potential at the point D exceeds the source voltage V.sub.CC. After a lapse of a constant period, no current flows through the control head coil 5, and the potentials at the points C and D substantially reach the source voltage V.sub.CC. Operation similar to the above is thereafter performed in sequence.
In the conventional control head driver of a video tape recorder having the aforementioned structure, back electromotive force is caused in the control head coil 5 by abrupt current change in switching of the control signal B, whereby the potential at an end of the control head coil 5 instantaneously drops to GND or exceeds the source voltage V.sub.CC. When this circuit is integrated, such instantaneous potential change at the end of the control head coil 5 exerts influence on another circuit part of the integrated circuit, to cause malfunction.